Polyatomic ion

A polyatomic ion, also known as a molecular ion, is a charged chemical species (ion) composed of two or more atoms covalently bonded or of a metal complex that can be considered to be acting as a single unit. The prefix poly- means "many," in Greek, but even ions of two atoms are commonly referred to as polyatomic. In older literature, a polyatomic ion is also referred to as a radical, and less commonly, as a radical group. In contemporary usage, the term radical refers to free radicals that are (not necessarily charged) species with an unpaired electron.

An example of a polyatomic ion is the hydroxide ion; consisting of one oxygen atom and one hydrogen atom, hydroxide has a charge of −1. Its chemical formula is OH
. An ammonium ion consists of one nitrogen atom and four hydrogen atoms: it has a charge of +1, and its chemical formula is NH+
4
.

Polyatomic ions are often useful in the context of acid-base chemistry or in the formation of salts. A polyatomic ion can often be considered as the conjugate acid or base of a neutral molecule. For example, the conjugate base of sulfuric acid (H2SO4) is the polyatomic hydrogen sulfate anion (HSO
4
). The removal of another hydrogen ion yields the sulfate anion (SO2−
4
).

Nitrate-ion-elpot
An electrostatic potential map of the nitrate ion (NO
3
). Areas coloured translucent red, around the outside of the red oxygen atoms themselves, signify the regions of most negative electrostatic potential.

Nomenclature of polyatomic anions

There are two "rules" that can be used for learning the nomenclature of polyatomic anions. First, when the prefix bi is added to a name, a hydrogen is added to the ion's formula and its charge is increased by 1, the latter being a consequence of the hydrogen ion's +1 charge. An alternative to the bi- prefix is to use the word hydrogen in its place: the anion derived from H+
+ CO2−
3
, HCO
3
, can be called either bicarbonate or hydrogencarbonate.

Most of the common polyatomic anions are oxyanions, conjugate bases of oxyacids (acids derived from the oxides of non-metallic elements). For example, the sulfate anion, SO2−
4
, is derived from H
2
SO
4
, which can be regarded as SO
3
+ H
2
O
.

The second rule looks at the number of oxygens in an ion. Consider the chlorine oxyanion family:

oxidation state −1 +1 +3 +5 +7
anion name chloride hypochlorite chlorite chlorate perchlorate
formula Cl
ClO
ClO
2
ClO
3
ClO
4
structure The chloride ion The hypochlorite ion The chlorite ion The chlorate ion The perchlorate ion

First, think of the -ate ion as being the "base" name, in which case the addition of a per- prefix adds an oxygen. Changing the -ate suffix to -ite will reduce the oxygens by one, and keeping the suffix -ite and adding the prefix hypo- reduces the number of oxygens by one more. In all situations, the charge is not affected. The naming pattern follows within many different oxyanion series based on a standard root for that particular series. The -ite has one less oxygen than the -ate, but different -ate anions might have different numbers of oxygen atoms.

These rules do not work with all polyatomic anions, but they do work with the most common ones. Following table give examples for some of these common anion groups.

bromide hypobromite bromite bromate perbromate
Br
BrO
BrO
2
BrO
3
BrO
4
Iodide Hypoiodite Iodite Iodate periodate
I
IO
IO
2
IO
3
IO
4
or IO5−
6
sulfide hyposulfite sulfite sulfate persulfate
S2−
S
2
O2−
2
SO2−
3
SO2−
4
SO2−
5
selenide hyposelenite selenite selenate
Se2−
Se
2
O2−
2
SeO2−
3
SeO2−
4
telluride hypotellurite tellurite tellurate
Te2−
TeO2−
2
TeO2−
3
TeO2−
4
nitride hyponitrite nitrite nitrate
N3−
N
2
O2−
2
NO
2
NO
3
phosphide hypophosphite phosphite phosphate perphosphate
P3−
H
2
PO
2
PO3−
3
PO3−
4
HPO2−
5
arsenide hypoarsenite arsenite arsenate
As3−
AsO3−
2
AsO3−
3
AsO3−
4

Other Examples of common polyatomic ions

The following tables give additional examples of commonly encountered polyatomic ions. Only a few representatives are given, as the number of polyatomic ions encountered in practice is very large.

Anions
Tetrahydroxyborate B(OH)
4
Acetylide C2−
2
Ethoxide or ethanolate C
2
H
5
O
Acetate or ethanoate CH
3
COO
or C
2
H
3
O
2
Benzoate C
6
H
5
COO
or C
7
H
5
O
2
Citrate C
6
H
5
O3−
7
Carbonate CO2−
3
Oxalate C
2
O2−
4
Cyanide CN
Chromate CrO2−
4
Dichromate Cr
2
O2−
7
Bicarbonate or hydrogencarbonate HCO
3
Hydrogen phosphate HPO2−
4
Dihydrogen phosphate H
2
PO
4
Hydrogen sulfate or bisulfate HSO
4
Manganate MnO2−
4
Permanganate MnO
4
Azanide or amide NH
2
Peroxide O2−
2
Hydroxide OH
Bisulfide SH
Thiocyanate SCN
Silicate SiO2−
4
Thiosulfate S
2
O2−
3
Cations
Onium ions Carbenium ions Others
Guanidinium C(NH
2
)+
3
Tropylium C
7
H+
7
Mercury(I) Hg2+
2
Ammonium NH+
4
Triphenylcarbenium (C
6
H
5
)
3
C+
Phosphonium PH+
4
cyclopropenium C
3
H+
3
Hydronium H
3
O+
Fluoronium H
2
F+
Pyrylium C
5
H
5
O+

See also

External links

Ammonium

The ammonium cation is a positively charged polyatomic ion with the chemical formula NH+4. It is formed by the protonation of ammonia (NH3). Ammonium is also a general name for positively charged or protonated substituted amines and quaternary ammonium cations (NR+4), where one or more hydrogen atoms are replaced by organic groups (indicated by R).

Arsonium

The arsonium cation is a positively charged polyatomic ion with the chemical formula AsH+4. An arsonium salt is a salt containing either the arsonium (AsH+4) cation, such as arsonium bromide (AsH+4Br−) and arsonium iodide (AsH+4I−), which can be synthesized by reacting arsine with hydrogen bromide or hydrogen iodide.. Or more commonly, as organic derivative such as the quaternary arsonium salts Ph4As+Cl− (CAS: [123334-18-9], hydrate form) and the zwitterionic compound arsenobetaine.

Bismuth polycations

Electron-deficient bismuth polycations are classical examples of homopolyatomic ions (a polyatomic ion composed entirely of a single element) composed of electron-deficient bismuth atoms. They were originally observed in dilute solutions of bismuth metal in molten bismuth chloride. It has since been found that these clusters are present in the solid state, particularly in salts where germanium tetrachloride or tetrachloroaluminate serve as the counteranions, but also in amorphous phases such as glasses and gels.. Bismuth endows materials with a variety of interesting optical properties that can be tuned by changing the supporting material. Commonly-reported structures include the trigonal bipyramidal Bi3+5 cluster, the octahedral Bi2+6 cluster, the square antiprismatic Bi2+8 cluster, and the tricapped trigonal prismatic Bi5+9 cluster.

Carbide

In chemistry, a carbide is a compound composed of carbon and a less electronegative element. Carbides can be generally classified by the chemical bonds type as follows: (i) salt-like, (ii) covalent compounds, (iii) interstitial compounds, and (iv) "intermediate" transition metal carbides. Examples include calcium carbide (CaC2), silicon carbide (SiC), tungsten carbide (WC; often called, simply, carbide when referring to machine tooling), and cementite (Fe3C), each used in key industrial applications. The naming of ionic carbides is not systematic.

Carbonate

In chemistry, a carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula of CO2−3. The name may also refer to a carbonate ester, an organic compound containing the carbonate group C(=O)(O–)2.

The term is also used as a verb, to describe carbonation: the process of raising the concentrations of carbonate and bicarbonate ions in water to produce carbonated water and other carbonated beverages – either by the addition of carbon dioxide gas under pressure, or by dissolving carbonate or bicarbonate salts into the water.

In geology and mineralogy, the term "carbonate" can refer both to carbonate minerals and carbonate rock (which is made of chiefly carbonate minerals), and both are dominated by the carbonate ion, CO2−3. Carbonate minerals are extremely varied and ubiquitous in chemically precipitated sedimentary rock. The most common are calcite or calcium carbonate, CaCO3, the chief constituent of limestone (as well as the main component of mollusc shells and coral skeletons); dolomite, a calcium-magnesium carbonate CaMg(CO3)2; and siderite, or iron(II) carbonate, FeCO3, an important iron ore. Sodium carbonate ("soda" or "natron") and potassium carbonate ("potash") have been used since antiquity for cleaning and preservation, as well as for the manufacture of glass. Carbonates are widely used in industry, e.g. in iron smelting, as a raw material for Portland cement and lime manufacture, in the composition of ceramic glazes, and more.

Chemical formula

A chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, such as parentheses, dashes, brackets, commas and plus (+) and minus (−) signs. These are limited to a single typographic line of symbols, which may include subscripts and superscripts. A chemical formula is not a chemical name, and it contains no words. Although a chemical formula may imply certain simple chemical structures, it is not the same as a full chemical structural formula. Chemical formulas can fully specify the structure of only the simplest of molecules and chemical substances, and are generally more limited in power than are chemical names and structural formulas.

The simplest types of chemical formulas are called empirical formulas, which use letters and numbers indicating the numerical proportions of atoms of each type. Molecular formulas indicate the simple numbers of each type of atom in a molecule, with no information on structure. For example, the empirical formula for glucose is CH2O (twice as many hydrogen atoms as carbon and oxygen), while its molecular formula is C6H12O6 (12 hydrogen atoms, six carbon and oxygen atoms).

Sometimes a chemical formula is complicated by being written as a condensed formula (or condensed molecular formula, occasionally called a "semi-structural formula"), which conveys additional information about the particular ways in which the atoms are chemically bonded together, either in covalent bonds, ionic bonds, or various combinations of these types. This is possible if the relevant bonding is easy to show in one dimension. An example is the condensed molecular/chemical formula for ethanol, which is CH3-CH2-OH or CH3CH2OH. However, even a condensed chemical formula is necessarily limited in its ability to show complex bonding relationships between atoms, especially atoms that have bonds to four or more different substituents.

Since a chemical formula must be expressed as a single line of chemical element symbols, it often cannot be as informative as a true structural formula, which is a graphical representation of the spatial relationship between atoms in chemical compounds (see for example the figure for butane structural and chemical formulas, at right). For reasons of structural complexity, a single condensed chemical formula (or semi-structural formula) may correspond to different molecules, known as isomers. For example glucose shares its molecular formula C6H12O6 with a number of other sugars, including fructose, galactose and mannose. Linear equivalent chemical names exist that can and do specify uniquely any complex structural formula (see chemical nomenclature), but such names must use many terms (words), rather than the simple element symbols, numbers, and simple typographical symbols that define a chemical formula.

Chemical formulas may be used in chemical equations to describe chemical reactions and other chemical transformations, such as the dissolving of ionic compounds into solution. While, as noted, chemical formulas do not have the full power of structural formulas to show chemical relationships between atoms, they are sufficient to keep track of numbers of atoms and numbers of electrical charges in chemical reactions, thus balancing chemical equations so that these equations can be used in chemical problems involving conservation of atoms, and conservation of electric charge.

Dihydroxymethylidene

Dihydroxymethylidene is a chemical compound with formula C(OH)2. It is an unstable tautomer of formic acid. There is no evidence that this compound exists in solution, but the molecule has been detected in the gas phase. Many related carbenes are known, although they are often transient.

Functional group

In organic chemistry, functional groups are specific substituents or moieties within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction(s) regardless of the size of the molecule it is a part of. This allows for systematic prediction of chemical reactions and behavior of chemical compounds and design of chemical syntheses. Furthermore, the reactivity of a functional group can be modified by other functional groups nearby. In organic synthesis, functional group interconversion is one of the basic types of transformations.

Functional groups are groups of one or more atoms of distinctive chemical properties no matter what they are attached to. The atoms of functional groups are linked to each other and to the rest of the molecule by covalent bonds. For repeating units of polymers, functional groups attach to their nonpolar core of carbon atoms and thus add chemical character to carbon chains. Functional groups can also be charged, e.g. in carboxylate salts (–COO−), which turns the molecule into a polyatomic ion or a complex ion. Functional groups binding to a central atom in a coordination complex are called ligands. Complexation and solvation are also caused by specific interactions of functional groups. In the common rule of thumb "like dissolves like", it is the shared or mutually well-interacting functional groups which give rise to solubility. For example, sugar dissolves in water because both share the hydroxyl functional group (–OH) and hydroxyls interact strongly with each other. Plus, when functional groups are more electronegative than atoms they attach to, the functional groups will become polar, and the otherwise nonpolar molecules containing these functional groups become polar and so become soluble in some aqueous environment.

Combining the names of functional groups with the names of the parent alkanes generates what is termed a systematic nomenclature for naming organic compounds. In traditional nomenclature, the first carbon atom after the carbon that attaches to the functional group is called the alpha carbon; the second, beta carbon, the third, gamma carbon, etc. If there is another functional group at a carbon, it may be named with the Greek letter, e.g., the gamma-amine in gamma-aminobutyric acid is on the third carbon of the carbon chain attached to the carboxylic acid group. IUPAC conventions call for numeric labeling of the position, e.g. 4-aminobutanoic acid. In traditional names various qualifiers are used to label isomers, for example, isopropanol (IUPAC name: propan-2-ol) is an isomer of n-propanol (propan-1-ol).

IUPAC nomenclature of inorganic chemistry

In chemical nomenclature, the IUPAC nomenclature of inorganic chemistry is a systematic method of naming inorganic chemical compounds, as recommended by the International Union of Pure and Applied Chemistry (IUPAC). It is published in Nomenclature of Inorganic Chemistry (which is informally called the Red Book). Ideally, every inorganic compound should have a name from which an unambiguous formula can be determined. There is also an IUPAC nomenclature of organic chemistry.

List of compounds

Compounds are organized into the following lists:

List of inorganic compounds, compounds without a C–H bond

List of biomolecules

Monatomic ion

A monatomic ion is an ion consisting of exactly one atom. If an ion contains more than one atom, even if these are of the same element, it is called a polyatomic ion. For example, calcium carbonate consists of the monatomic ion Ca2+ and the polyatomic ion CO32−.

A type I binary ionic compound contains a metal (cation) that forms only one type of ion. A type II ionic compound contains a metal that forms more than one type of ion, i.e., ions with different charges.

Nitrate

Nitrate is a polyatomic ion with the molecular formula NO−3 and a molecular mass of 62.0049 u. Organic compounds that contain the nitrate ester as a functional group (RONO2) are also called nitrates.

Oxycation

An oxycation is a polyatomic ion with a positive charge that contains oxygen.

Platinocyanide

Platinocyanide, also known as tetracyanoplatinate (IUPAC), cyanoplatinate, or platinocyanate, is a polyatomic ion with the molecular formula [Pt(CN)4]2−. The name also applies to compounds containing this ion, which are salts of the hypothetical platinocyanic acid (sometimes platinocyanhydric acid).

Barium platinocyanide, Ba[Pt(CN)4] is a phosphor and a scintillator. It fluoresces in the presence of x-rays and gamma rays. It was important in the discovery of X-rays, and in the development of the fluoroscope.

One platinocyanide salt, Krogmann's salt (dipotassium tetracyanoplatinate bromide trihydrate), has unusually high electric conductance.

Polyoxometalate

In chemistry, a polyoxometalate (abbreviated POM) is a polyatomic ion, usually an anion, that consists of three or more transition metal oxyanions linked together by shared oxygen atoms to form closed 3-dimensional frameworks. The metal atoms are usually group 6 (Mo, W) or less commonly group 5 (V, Nb, Ta) transition metals in their high oxidation states. They are usually colorless or orange, diamagnetic anions. Two broad families are recognized, isopolymetalates, composed of only one kind of metal and oxide, and heteropolymetalates, composed of one metal, oxide, and a main group oxyanion (phosphate, silicate, etc.). Many exceptions to these general statements exist.

Tetrafluoroammonium

The tetrafluoroammonium cation (also known as perfluoroammonium) is a positively charged polyatomic ion with chemical formula NF+4. It is equivalent to the ammonium ion where the hydrogen atoms surrounding the central nitrogen atom have been replaced by fluorine. Tetrafluoroammonium ion is isoelectronic with tetrafluoromethane CF4 and the tetrafluoroborate BF−4 anion.

The tetrafluoroammonium ion forms salts with a large variety of fluorine-bearing anions. These include the bifluoride anion (HF−2), tetrafluorobromate (BrF−4), metal pentafluorides (MF−5 where M is Ge, Sn, or Ti), hexafluorides (MF−6 where M is P, As, Sb, Bi, or Pt), heptafluorides (MF−7 where M is W, U, or Xe), octafluorides (XeF2−8), various oxyfluorides (MF5O− where M is W or U; FSO−3, BrF4O−), and perchlorate (ClO−4). Attempts to make the nitrate salt, NF4NO3, were unsuccessful because of quick fluorination: NF+4 + NO−3 → NF3 + FONO2.

Triflate

Triflate, also known by the systematic name trifluoromethanesulfonate, is a functional group with the formula CF3SO3−. The triflate group is often represented by −OTf, as opposed to −Tf (triflyl). For example, n-butyl triflate can be written as CH3CH2CH2CH2OTf.

The corresponding triflate anion, CF3SO−3, is an extremely stable polyatomic ion, being the conjugate base of triflic acid (CF3SO3H), one of the strongest acids known. It is defined as a superacid, because it is more acidic than pure sulfuric acid.

Trifluorooxonium

The trifluorooxonium cation is a hypothetical positively charged polyatomic ion with chemical formula OF+3. It is structurally equivalent to the hydronium ion where the hydrogen atoms surrounding the central oxygen atom have been replaced by fluorine, and is isoelectronic with nitrogen trifluoride. This cation would be an example of oxygen in the +4 oxidation state.

The OF+3 cation was shown to be vibrationally stable at all levels of theory applied (HF, MP2, CCSD(T)). OF+3 was proposed to possess a pyramidal structure with an O–F bond length of 1.395 Å and F–O–F bond angles of 104.2° (CCSD(T) level of theory). The F+ detachment energy of the OF+3 cation was calculated to be +110.1 kcal mol−1. However, the low-temperature reaction of F2, OF2 and AsF5 under UV irradiation, besides unreacted starting materials only yielded the dioxygenyl salt O+2[AsF6]−. The oxidation of OF2 with KrF+ salts also failed to produce evidence for the title cation.The formation of the hypothetical salt OF+3[AsF6]− was calculated to be about thermoneutral, but slightly unfavorable with OF2(g) + F2(g) + AsF5(g) → OF+3[AsF6]−(s) = +10.5 kcal mol−1.

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